Reason. Marcus Aurelius 5.14

Reason's power lies in seeing the end from the beginning: it measures our life in terms of foreseen mortality. As we act and accumulate experience with living, our reason naturally marks how we should behave in future to avoid suffering our mortality too soon. But we know that we must suffer it eventually, and then it will be impossible to avoid or overcome. The precise method and manner of our final encounter with death are unknown, irrational, but the best practical means of approaching that encounter will be available to our reason as memories of past actions undertaken and finished. When we meet death the last time, it will not be the first time we have ever seen him. We will be able to surrender and end our journey at peace, recognizing the rational kinship the unites this final end with all the other ones before it. That is how I read the following meditation from Marcus.

Ὁ λόγος καὶ ἡ λογικὴ τέχνη δυνάμεις εἰσὶν ἑαυταῖς ἀρκούμεναι καὶ τοῖς καθ’ αὑτὰς ἔργοις. ὁρμῶνται μὲν οὖν ἀπὸ τῆς οἰκείας ἀρχῆς, ὁδεύουσι δὲ εἰς τὸ προκείμενον τέλος, καθὸ κατορθώσεις αἱ τοιαῦται πράξεις ὀνομάζονται τὴν ὀρθότητα τῆς ὁδοῦ σημαίνουσαι.

Reason and the rational arts are faculties sufficient to look after themselves, and the works that require them. Beginning from the seat of power that is their native abode, they make their way to ends foreknown. Thus rational actions are called right or true, because they set proper waymarks on life's journey.